Absolute distance meter that measures a moving retroreflector

ABSTRACT

A laser device and method capable of one or more dimensional absolute distance measurements and/or surface scanning and/or coordinate measurements of a moving external retroreflector or other moving target surfaces without using an incremental interferometer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.11/239,854, filed on Sep. 30, 2005, which claims priority to ProvisionalPatent Application No. 60/614,778, filed on Sep. 30, 2004, both of whichare incorporated by reference.

BACKGROUND

The present disclosure relates to a coordinate measuring device. One setof coordinate measurement devices belongs to a class of instruments thatmeasure the coordinates of a point by sending a laser beam to the point.The laser beam may impinge directly on the point or may impinge on aretroreflector target that is in contact with the point. In either case,the instrument determines the coordinates of the point by measuring thedistance and the two angles to the target. The distance is measured witha distance-measuring device such as an absolute distance meter or aninterferometer. The angles are measured with an angle-measuring devicesuch as an angular encoder. A gimbaled beam-steering mechanism withinthe instrument directs the laser beam to the point of interest.Exemplary systems for determining coordinates of a point are describedby U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339Lau et al.

The laser tracker is a particular type of coordinate-measuring devicethat tracks the retroreflector target with one or more laser beams itemits. A device that is closely related to the laser tracker is thelaser scanner. The laser scanner steps one or more laser beams to pointson a diffuse surface. The laser tracker and laser scanner are bothcoordinate-measuring devices. It is common practice today to use theterm laser tracker to also refer to laser scanner devices havingdistance- and angle-measuring capability. This broad definition of lasertracker, which includes laser scanners, is used throughout thisapplication.

One type of laser tracker contains only an interferometer without anabsolute distance meter. If an object blocks the path of the laser beamfrom one of these trackers, the interferometer loses its distancereference. The operator must then track the retroreflector to a knownlocation before continuing the measurement. A way around this limitationis to put an absolute distance meter (ADM) in the tracker. The ADM canmeasure distance in a point-and-shoot manner. Some laser trackerscontain only an ADM without an interferometer. An exemplary lasertracker of this type is described in Payne, et al U.S. Pat. No.5,455,670. Other laser trackers typically contain both an ADM and aninterferometer. An exemplary laser tracker of this type is described inMeier, et al. U.S. Pat. No. 5,764,360.

One of the main applications for laser trackers is to scan the surfacefeatures of objects to determine their geometrical characteristics. Forexample, an operator can determine the angle between two surfaces byscanning each of the surfaces and then fitting a geometrical plane toeach. As another example, an operator can determine the center andradius of a sphere by scanning the sphere surface. Up until this time,an interferometer, rather than an ADM, has been required for the lasertracker to scan. The reason for this is that absolute distancemeasurements have only been possible on stationary targets.Consequently, to get full functionality with both scanning andpoint-and-shoot capability, laser trackers have required both aninterferometer and an ADM. What is needed is an ADM that has the abilityto accurately and quickly scan a moving target. This permits trackercost to be reduced because the interferometer is no longer needed.

SUMMARY

The above and other problems and disadvantages of the prior art areovercome and alleviated by embodiments the present laser device, whichutilizes an absolute distance meter to determine the distance to amoving retroreflector.

A laser device and method is disclosed capable of one or moredimensional absolute distance measurements and/or surface scanningand/or coordinate measurements of a moving external retroreflector orother moving target surfaces without using an incremental interferometerdepending upon what the application requires.

The above-discussed and other features and advantages of the presentapparatus and method will be appreciated and understood by those skilledin the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is a perspective view of an exemplary laser tracker sending alaser beam to an external retroreflector; and

FIG. 2 is a block diagram of some of the main elements within theexemplary laser tracker of FIG. 1; and

FIG. 3 is a block diagram of the elements within the exemplaryfiber-coupling network of FIG. 2; and

FIG. 4 is a block diagram of the elements within the exemplary ADMelectronics of FIG. 2; and

FIG. 5 is a block diagram of the elements within an exemplary ADMdata-processing system for computing the distance to a movingretroreflector.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings.

An exemplary laser tracker 10 is illustrated in FIG. 1. An exemplarygimbaled beam-steering mechanism 12 of the laser tracker compriseszenith carriage 14 that is mounted on azimuth base 16. The zenith andazimuth mechanical axes internal to the tracker (not shown) are turnedto point the laser beam 46 in the desired direction. The laser beam maycomprise one or more laser wavelengths, as will be described in thediscussion that follows. The zenith and azimuth angular encodersinternal to the tracker (not shown) are attached to the zenith andazimuth mechanical axes and indicate, to high accuracy, the angles ofrotation. For the sake of clarity and simplicity, this sort of gimbalmechanism 12 is assumed in the following discussion. However, othertypes of gimbal mechanisms are possible, and the techniques describedhere are also applicable to these other types.

Laser beam 46 travels to external retroreflector 26. The most commontype of retroreflector is a spherically mounted retroreflector (SMR),which comprises a metal sphere into which a cube-corner retroreflector(not shown) is embedded. The cube-corner retroreflector comprises threeperpendicular mirrors that come together at a common apex point. Theapex point is placed at the center of the metal sphere. Instead of anSMR, a retrosphere or any other device that sends the return laser beamback on itself may be used as the external retroreflector 26.

Elements of the Laser Tracker

Some of the main elements within the laser tracker are shown in FIG. 2.ADM electronics 300 modulates the optical power of ADM laser 102, whichsends light through fiber-optic cable 104 and fiber-coupling network200. Some of the light from the fiber-coupling network 200 travels toADM beam launch 140. Another part of the light travels through fiberloop 106 and then back into fiber-coupling network 200. ADM beam launch140 comprises stable ferrule 142 and positive lens 144. Collimated light108 emerges from the fiber launch 140.

In the event that the ADM laser operates at an infrared wavelength, itis convenient to provide a visible laser beam to help make the ADM beameasier to find. Visible-light laser 110 sends visible light into beamlaunch 150, which comprises stable ferrule 152 and positive lens 154.The visible laser beam 112 that emerges to the beam launch 150 iscollimated. Dichroic beam splitter 114 transmits ADM beam 108 butreflects visible beam 112. To the right of beam splitter 114, compositelaser beam 116 comprises the visible laser beam and ADM laser beam,which are substantially collinear. Laser beam 116 passes through beamsplitter 118 and beam expander 160, emerging as a larger collimatedlaser beam 46. The beam expander comprises negative lens 162 andpositive lens 164.

The laser beam 46 travels to external retroreflector 26, as shown inFIG. 1. The beam reflects off retroreflector 26 and returns to the lasertracker. If the laser beam strikes the center of the retroreflector, thereflected laser beam retraces the path of the incident laser beam. Ifthe laser beam strikes the retroreflector off the center, the reflectedlaser beam returns parallel to the incident beam but offset from it. Thereturning laser beam re-enters the tracker and retraces the path backthrough the optical system. Some of the returning laser light reflectsoff beam splitter 118. Reflected laser light 126 passes through opticalfilter 128 and strikes position detector 130. The optical filter 128blocks either the ADM light or the visible light in the beam 126. Theposition detector 130 responds to the light that passes through theoptical filter 128 by indicating the position of the laser beam on itssurface. The retrace point of the position detector is defined as thepoint that the laser beam 126 strikes if the beam 46 strikes the centerof retroreflector 26. When the laser beam 46 moves off the center ofretroreflector 26, the laser beam 126 moves off the retrace point andcauses the position detector 130 to generate an electrical error signal.A servo system processes this error signal to activate motors that turnthe laser tracker toward the center of the external retroreflector 26.

The dichroic beam splitter 114 reflects the returning visible laser beambut transmits the returning ADM laser beam. The returning ADM laser beamtravels through the beam launch and is coupled into the optical fiberwithin the stable ferrule 142. This light travels through thefiber-coupling network 200 and emerges from optical fiber 230. Thatportion of the laser light that traveled through fiber loop 106 emergesfrom optical fiber 232. Both fibers 230 and 232 continue into the ADMelectronics section 300, where their modulated powers are converted intoelectrical signals. These signals are processed by the ADM electronicsto provide the result, which is the distance from the tracker to theretroreflector target.

Fiber-coupling Network

Exemplary fiber-coupling network 200 of FIG. 3 comprises firstfiber-optic coupler 204, second fiber-optic coupler 206, andlow-reflection terminations 208 and 210. Light from ADM laser 102travels through fiber-optic cable 104 and enters first fiber-opticcoupler 204. Fiber-optic coupler 204 sends 10% of the laser lightthrough fiber-loop 106 and into optical fiber 232, which travels to ADMelectronics 300. Fiber-optic coupler 204 sends the other 90% of thelaser light through fiber-optic coupler 206, which sends half of thelaser light to low-reflection termination 208 and the other half of thelaser light to stable ferrule 142. Light from stable ferrule 142propagates to external retroreflector 26 and back into ferrule 142, asdescribed above. Half of the laser light returning through ferrule 142travels back through second fiber-optic coupler 206, through fiber-opticcable 230, and into ADM electronics 300. The other half of the laserlight travels through second fiber-coupler 206, first fiber-opticcoupler 204, and into ADM laser 102, where it is blocked by an internalFaraday isolator (not shown).

ADM Electronics

ADM electronics 300 of FIG. 4 comprises frequency reference 302,synthesizer 304, measure detector 306, reference detector 308, mixers310, 312, amplifiers 314, 316, 318, 320, frequency divider 324, andanalog-to-digital converter (ADC) 322. Frequency reference 302 providesthe time base for the ADM and should have low phase noise and lowfrequency drift. The frequency reference may be an oven-controlledcrystal oscillator (OCXO), rubidium oscillator, or any other highlystable frequency reference. Preferably the oscillator frequency shouldbe accurate and stable to within a small fraction of a part per million.The signal from the frequency reference is put into the synthesizer,which generates three signals. The first signal is at frequency f_(RF)and modulates the optical power of ADM laser 102. This type ofmodulation is called intensity modulation (IM). Alternatively, it ispossible for the first signal at frequency f_(RF) to modulate theelectric field amplitude, rather than the optical power, of the laserlight from ADM laser 102. This type of modulation is called amplitudemodulation (AM). The second and third signals, both at the frequencyf_(LO), go to the local-oscillator ports of mixers 310 and 312.

Fiber-optic cables 230 and 232 carry laser light. The light in thesefiber-optic cables is converted into electrical signals by measuredetector 306 and reference detector 308. These optical detectors sendthe modulation frequency f_(RF) to amplifiers 314, 316 and then tomixers 310, 312. Each mixer produces two frequencies, one at|f_(LO)−f_(RF)| and one at |f_(LO)+f_(RF)|. These signals travel tolow-frequency amplifiers 318, 320. These amplifiers block thehigh-frequency signals so that only the signals at the intermediatefrequency (IF), f_(IF)=|f_(LO)−f_(RF)| pass through to theanalog-to-digital converter (ADC) 322. The frequency reference 302 sendsa signal into frequency divider 324, which divides the frequency of thereference 302 by an integer N to produce a sampling clock. In general,the ADC may decimate the sampled signals by an integer factor M, so thatthe effective sampling rate is f_(REF)/NM. This effective sampling rateshould be an integer multiple of the intermediate frequency f_(IF).

Here are frequencies for an exemplary ADM: The frequency reference isf_(REF)=20 MHz. The synthesizer RF frequency that drives the laser isf_(RF)=2800 MHz. The synthesizer LO frequency that is applied to themixers is f_(LO)=2800.01 MHz. The difference between the LO and RFfrequencies is the intermediate frequency of f_(IF)=10 kHz. Thefrequency reference is divided by N=10, to produce a 2-MHz frequencythat is applied to the ADC as a sampling clock. The ADC has a decimationfactor of M=8, which produces an effective sampling rate of 250 kHz.Since the IF is 10 kHz, the ADC takes 25 samples per cycle.

The ADC sends the sampled data for the measure and reference channels todata processors 400 for analysis. Data processors include digital signalprocessor (DSP) chips and general-purpose microprocessor chips. Theprocessing performed by these processors is described below.

Data Processor

Data processor 400 of FIG. 5 takes the digitized data from ADC 322 andderives from it the distance from the tracker to external retroreflector26. FIG. 5 refers to this distance as the RESULT. Data processor 400comprises digital signal processor 410, microprocessor 450, and crystaloscillators 402, 404.

Analog-to-digital converter 322 sends sampled data to DSP 410. This datais routed to a program that runs within the DSP. This program containsthree main functions: phase-extractor function 420, compensator function422, and Kalman-filter function 424. The purpose of the phase-extractorfunction is to determine the phases of the signals in the reference andmeasure channels, that is, the phases of the signals that pass throughthe measure detector 306 and reference detector 308. To determine thesephases, the modulation range must first be calculated. Modulation rangeis defined as the round-trip distance traveled by the ADM laser light inair for the phase of the laser modulation to change by 2 pi radians. Themodulation range R_(MOD) is given byR _(MOD) =c/(2nf _(RF)),  (1)where c is the speed of light in vacuum, n is the group index ofrefraction of the ADM laser light in air, and f_(RF) is the RF frequencygenerated by synthesizer 304 and applied to ADM laser 102. In anexemplary ADM having an RF frequency of 2860 MHz, the modulation rangeis approximately 52 millimeters.

As discussed previously, the sample clock applied to ADC 322 has aneffective frequency of f_(SAMP)=f_(REF)/NM and the number of ADC samplesper cycle is V=f_(SAMP)/f_(IF). In an exemplary tracker, f_(REF)=20 MHz,N=10, M=8, and f_(IF)=10 kHz. The sample frequency is then 250 kHz andthe number of ADC samples per cycle is N_(ADC)=25 samples per cycle.

Let x_(k) be the k^(th) sampled data value from the ADC for the measurechannel and let v be the corresponding speed of external retroreflector26 during the measurement. Phase-extractor function 420 calculates thephase PM of the measure channel for moving external retroreflector 26 asfollows:

$\begin{matrix}{{a = {\sum\limits_{k = 0}^{V - 1}{x_{k}{\sin\left( {2\pi\; k\;\frac{f_{IF} - {v/R_{MOD}}}{f_{SAMP}}} \right)}}}},} & (2) \\{{b = {\sum\limits_{k = 0}^{V - 1}{x_{k}{\cos\left( {2\pi\; k\;\frac{f_{IF} - {v/R_{MOD}}}{f_{STAMP}}} \right)}}}},} & (3) \\{p_{M} = {{\tan^{- 1}\left( {a\text{/}b} \right)}.}} & (4)\end{matrix}$Let y_(k) be the k^(th) sampled data values from the ADC for thereference channel. Phase-extractor function 420 calculates the phasep_(R) of the reference channel for moving external retroreflector 26 asfollows:

$\begin{matrix}{{g = {\sum\limits_{k = 0}^{V - 1}{y_{k}{\sin\left( {2\pi\; k\;\frac{f_{IF}}{f_{SAMP}}} \right)}}}},} & (5) \\{{h = {\sum\limits_{k = 0}^{V - 1}{y_{k}{\cos\left( {2\pi\; k\;\frac{f_{IF}}{f_{SAMP}}} \right)}}}},} & (6) \\{p_{R} = {{\tan^{- 1}\left( {g\text{/}h} \right)}.}} & (7)\end{matrix}$

Significantly, the phase-extractor function 420 is dependent on thespeed or velocity v, for example the radial speed, of the target as showin equation (2), (3), (5), and (6). The phase-extractor function 420also delivers the measure phase PM and the reference phase p_(R) to thecompensator function, which uses these phases to calculate a distancevalue:d=d ₀ +R _(MOD) [W+(p _(M) −p _(R))/2π].  (8)The quantity W is an integer that accounts for the number of wholemodulation intervals to the target. The method for finding this integeris discussed below. In some systems, there may be additional systematicerrors that can be removed by appending additional terms to equation(8). For example, some systems experience an error that varies withdistance as a sinusoid with a period equal to the modulation rangeR_(MOD). To remove this type of error, it is necessary to use the ADM tomeasure targets at accurately known distances and observe the sinusoidalerror pattern.

The compensator 422 sends the distance values to Kalman filter 424. TheKalman filter is a numerical algorithm applied to the distance data togive the best estimate of distance and speed of external retroreflector26 as a function of time and in the presence of noise. The ADM distancedata is collected at high speed and has some level of random noise inthe distance readings. This small error is greatly amplified incalculating speed, since small differences in distance are divided by asmall increment in time. The Kalman filter can be thought of as anintelligent smoothing function that optimizes accuracy based on thenoise of the system and the speed of the target.

The Kalman filter also serves to synchronize the ADM readings with thereadings of the angular encoders and the position detector. The angularencoders and position detector latch their readings whenever theyreceive the sync pulse, which occurs at frequency f_(SYNC). Thefrequency of the sync pulse is in general different than the frequencyof calculation of the ADM. In an exemplary tracker, the ADM calculatesat a rate of f_(IF)=10 kHz, while the sync pulse has a frequency of1.024 kHz. The Kalman filter provides synchronization of the ADM withthe angular encoders and position detector by extrapolating the positionforward in time to the next sync pulse.

There are five general equations that govern the behavior of the Kalmanfilter. In general, the quantities in these equations are represented bymatrices, whose dimensions are determined by the complexity of theimplementation of the Kalman filter. The five general equations arex_(m)=Φx_(p),  (9)P _(m) =ΦP _(p)Φ^(T) +Q,  (10)K=P _(m) H ^(T)(HP _(m) H ^(T) +R)⁻¹,  (11)x _(p) =x _(m) +K(z−Hx _(m)),  (12)P _(p)=(P _(m) ⁻¹ +H ^(T) R ⁻¹ H)⁻¹.  (13)

In these equations, the subscript m represents an a priori estimate andthe subscript p represents an a posteriori estimate. The quantity x isthe state variable that may take a variety of forms. Because theexemplary ADM system measures at a high rate, a relatively simple statevector containing only two components—the position d and radial speedv—are needed:

$\begin{matrix}{x = {\begin{pmatrix}d \\v\end{pmatrix}.}} & (14)\end{matrix}$The corresponding time propagation matrix, assuming unit time steps, is

$\begin{matrix}{\Phi = {\begin{pmatrix}1 & 1 \\0 & 1\end{pmatrix}.}} & (15)\end{matrix}$Equation (9) then corresponds to the equations d_(m)=d_(p)+v_(p), whichmeans that the estimated distance at the present point in time (d_(m))is equal to the estimated distance at the last point in time (d_(p))times the estimated speed at the last point in time (v_(p)) times thetime interval between the current and last points in time, which isassumed to equal one. The matrix Q is the process noise covariance. Inthe simple Kalman filter employed here, the acceleration is notexplicitly calculated. Instead the acceleration is assumed to have adispersion characterized by the variance σ_(A) ². The process-noisevariance σ_(A) ² is selected so as to minimize the error in the positionof a moving target. The resulting covariance for the process noise is

$\begin{matrix}{Q = {{\sigma_{A}^{2}\begin{pmatrix}{1/4} & {1/2} \\{1/2} & 1\end{pmatrix}}.}} & (16)\end{matrix}$P_(m) is the state covariance matrix at the present point in time. It isfound from the state covariance matrix at the last point in time and theprocess noise covariance. The state covariance matrix and themeasurement noise covariance R are used to determine the filter gain K.In the simple case considered here, the measurement noise covariance isjust the variance σ_(M) ² in readings caused by noise in the measurementdevice. In this case, the measurement noise in the ADM system isdetermined by simply calculating the variance σ_(ADM) ² in the distancesreported while the ADM is measuring a stationary target. H is themeasurement matrix, which is defined such that H times the stateestimate x is equal to the estimated output, against which measuredoutput, is compared. In the case considered here the measurements are ofthe distance d and so H=(1 0).

Equation (12) is interpreted as follows. x_(m) is the initial guess forthe state vector (distance and radial speed) based on the distance andradial speed for the previous point in time. The quantity z is themeasured distance d and Hx_(m) is the estimated distance. The quantityz−Hx_(m) is the difference between the measured and estimated distances.This difference is multiplied by the gain matrix K to provide anadjustment to the initial estimate x_(m) for the state matrix. In otherwords, the best estimate for the distance is a value between themeasured distance and the estimated distance. Equation (12) provides amathematically sound method of selecting the best (a posteriori)estimate of the distance and radial speed. Equation (13) provides a newestimate for the state covariance P_(p) at the next point in time.Equations (9)-(13) are solved each time compensator function 422 sends anew measured value to the Kalman filter.

To synchronize the ADM measurement to the measurements of the angularencoders and position detector, counter 414 determines the difference intime between the sync pulse and the last state distance. It does this inthe following way. Crystal oscillator 404 sends a low-frequency sinewave to frequency divider 452, located within microprocessor 450. Thisclock frequency is divided down to f_(SYNC), the frequency of the syncpulse. The sync pulse is sent over device bus 72 to DSP 410, angularencoder electronics 74, and position-detector electronics 76. In anexemplary system, the oscillator sends a 32.768 kHz signal throughfrequency divider 452, which divides by 32 to produce a sync-pulsefrequency f_(SYNC)=1.024 kHz. The sync pulse is sent to counter 414,which resides within DSP 410. The counter is clocked by crystal 402,which drives a phase-locked loop (PLL) device 412 within the DSP. In theexemplary system, oscillator 402 has a frequency of 30 MHz and PLL 412doubles this to produce a clock signal of 60 MHz to counter 414. Thecounter 414 determines the arrival of the sync pulse to a resolution of1/60 MHz=16.7 nanoseconds. The phase-extractor function 420 sends asignal to the counter when the ADC 322 has sent all the samples for onecycle. This resets counter 414 and begins a new count. The sync pulsestops the counting of counter 412. The total number of counts is dividedby the frequency to determine the elapsed time. Since the time intervalin the above equations was set to one, the normalized time intervalt_(NORM) is the elapsed time divided by the time interval. The statedistance X_(EXT) extrapolated to the sync pulse event isx _(EXT) =x _(k) +v _(k) t _(NORM.)  (17)The Kalman-filter function 424 provides the result, which is thedistance from the tracker to external retroreflector 26. The Kalmanfilter also provides the velocity to phase-extractor function 420 toapply in equations (2), (3), (5), and (6).

Previously the quantity W was introduced in equation (8) as an integerthat accounts for the number of whole modulation intervals to thetarget. This integer value W is found by first measuring the distance tothe external retroreflector 26. The frequencies f_(RF) and f_(LO) arechanged by a fixed amount and the distances are again measured. If theRF frequencies before and after the change are f₁ and f₂ and the phasedifference between the two measurements is p then the integer W is equalto the integer portion of (p/2π)(f₁/|f₂−f₁|). This technique will workout to a range of (c/2n)/(f₂−f₁). For example, if f₁ and f₂ differ by2.5 MHz, and if the f₁ is 2800 MHz, then the technique will work out toabout 60 meters. If desired, a third frequency can be added to assist indetermining the value of the integer W. Once W has been determined, itis not necessary to switch the frequencies again unless the beam isbroken. If the ADM continues to measure the external retroreflector 26without interruption, then it can easily keep track of the changes inthe integer W.

It will be apparent to those skilled in the art that, while exemplaryembodiments have been shown and described, various modifications andvariations can be made to the apparatus and method of measuring a movingretroreflector with an absolute distance meter disclosed herein withoutdeparting from the spirit or scope of the invention. Accordingly, it isto be understood that the various embodiments have been described by wayof illustration and not limitation.

1. A laser device capable of one dimensional absolute distancemeasurement of a moving external retroreflector or other moving targetsurfaces without using an incremental interferometer comprising: asource of laser light that is amplitude or intensity modulated and sentto and returned from the moving external retroreflector or other movingtarget surfaces to the laser device along a measurement path; anopto-electronic component to convert the laser light returned from theretroreflector or target surfaces along the measurement path into anelectrical signal; conditioning electronics for conditioning the firstelectrical signal to create a second electrical signal; digitizingelectronics to produce digitized values of the second electrical signal;a digital signal processor for receiving the digitized values of thesecond electrical signal wherein the digital signal processor comprisesat least a phase extractor module structured to execute a velocitydependant phase extractor function upon the digitized values and whereinthe digital signal processor calculates an absolute distance d to themoving retroreflector or other target moving at a velocity v.
 2. Thelaser device of claim 1 further comprising: a position detector forassisting in aiming the laser tracker by measuring the position of thereturned laser light from the moving external retroreflector or othermoving target surfaces.
 3. The laser device of claim 2 furthercomprising: reference beam optical components for directing part of thelaser light to the measurement path containing the moving externalretroreflector or other moving target surfaces and part of the laserlight to a separate reference path.
 4. The laser device of claim 3further comprising opto-electronic components to convert said laserlight in said measurement path into an RF measurement signal and toconvert said laser light in said reference path into an RF referencesignal, where said RF measurement signal and said RF reference signalare electrical signals that retain modulation at a first frequencyf_(RF); and further comprising additional conditioning electronics forconditioning the RF measurement signal to create an IF measurementsignal and for conditioning the RF reference signal to create an IFreference signal.
 5. The laser device of claim 4 further comprisingadditional digitizing electronics to produce digitized values of said IFmeasurement signal and IF reference signal at a third frequencyf_(SAMP), which is a multiple of second frequency f_(IF) and wherein thedata processor calculates the absolute distance d to the retroreflectoror other moving target surface moving at velocity v in air having indexof refraction n, speed in vacuum c, and integer ratio V=f_(SAMP)/f_(IF)from said digitized values X_(k) of said IF measurement signal and saiddigitized values Y_(k) of said IF reference signal using the followingformulas,${R_{MOD} = {c/\left( {2{nf}_{RF}} \right)}},{a = {\sum\limits_{k = 0}^{V - 1}{x_{k}{\sin\left( {2\pi\; k\;\frac{f_{IF} - {v/R_{MOD}}}{f_{SAMP}}} \right)}}}},{b = {\sum\limits_{k = 0}^{V - 1}{x_{k}{\cos\left( {2\pi\; k\;\frac{f_{IF} - {v/R_{MOD}}}{f_{SAMP}}} \right)}}}},{p_{M} = {\tan^{- 1}\left( {a\text{/}b} \right)}},\begin{matrix}{{g = {\sum\limits_{k = 0}^{V - 1}{y_{k}{\sin\left( {2\pi\; k\;\frac{f_{IF}}{f_{SAMP}}} \right)}}}},} \\{{h = {\sum\limits_{k = 0}^{V - 1}{y_{k}{\cos\left( {2\pi\; k\;\frac{f_{IF}}{f_{SAMP}}} \right)}}}},} \\{{p_{R} = {\tan^{- 1}\left( {g\text{/}h} \right)}},{d = {d_{0} + {R_{MOD}\left\lbrack {W + {{\left( {p_{M} - p_{R}} \right)/2}\pi}} \right\rbrack}}},}\end{matrix}$ where d₀ is a constant and W is an integer that accountsfor the number of whole modulation lengths R_(MOD) to saidretroreflector.
 6. The laser device of claim 1 further comprising: abeam-steering mechanism that directs the laser light sent out of thelaser tracker; and a position detector that monitors the position of thelaser light relative to a retrace point on the position detector andwherein the beam-steering mechanism adjusts the direction of the laserlight according to position data from the position detector.
 7. Thelaser device of claim 1 further comprising synchronization electronicswhich determine a timing of absolute distance measurements relative toan electrical synchronization signal.
 8. The laser device of claim 7wherein the digital signal processor processes a Kalman filter tosynchronize the absolute distance measurements with position detectormeasurements from a position detector and to provide an estimation ofdistance and speed of the moving external retroreflector or other movingtarget surfaces as a function of time and in the presence of noise.
 9. Alaser device capable of one dimensional absolute distance measurement ofa moving external retroreflector or other moving target surfaces withoutusing an incremental interferometer comprising: a source of laser lightthat is amplitude or intensity modulated and sent to and returned fromthe moving external retroreflector or other moving target surfaces tothe laser device along a measurement path; an opto-electronic componentto convert the laser light returned from the retroreflector or targetsurfaces along the measurement path into an electrical signal;digitizing electronics to produce digitized values of the electricalsignal; a position detector; a digital signal processor for receivingthe digitized values of the electrical signal wherein the digital signalprocessor calculates an absolute distance d to the moving retroreflectoror other target moving at a velocity v and executes a velocity dependantphase extractor function upon the digitized values; and wherein thedigital signal processor during the calculation also processes a Kalmanfilter to synchronize the absolute distance d measurements with positiondetector measurements from the position detector and to provide anestimation of distance and speed of the moving external retroreflectoror other moving target surfaces as a function of time and in thepresence of noise.
 10. A method capable of one dimensional absolutedistance measurement of a moving external retroreflector or other movingtarget surfaces without using an incremental interferometer comprising:sending and returning a source of laser light that is amplitude orintensity modulated and sent to and returned from the moving externalretroreflector or other moving target surfaces to the laser device alonga measurement path; converting the laser light returned from theretroreflector or target surfaces along the measurement path into afirst electrical signal; conditioning the first electrical signal tocreate a second electrical signal; digitizing values of the secondelectrical signal; receiving the digitized values of the secondelectrical signal; executing a velocity dependant phase extractorfunction upon the digitized values; and calculating an absolute distanced to the moving retroreflector or other target moving at a velocity v.